Individuals are at greatest risk for developing an alcohol use disorder (AUD) during late adolescence (Kandel and Logan, 1984; Brown et al., 2008), possibly due to the relative impulsiveness of late adolescents/young adults. Kandel and Logan (1984) have suggested that the decline in heavy alcohol use that typically occurs in the mid-twenties may reflect a maturational or developmental process, as the development of frontal structures implicated in self-regulation and impulse control is complete in humans around the early- to-mid twenties (Giedd, 2004; Hooper et al., 2004; Rubia et al., 2006; Eshel et al., 2007). Although there is general acceptance of the idea that frontal circuits are still maturing in late adolescents no work to date has specifically investigated changes in the function of frontal circuits engaged during Now versus Later decision making, a quantifiable measure of impulsiveness. The proposed studies will compare late adolescents (ages 18-23) and adults (ages 25-40) using functional MRI approaches to identify differences in the brain areas engaged during Now versus Later decision-making. In addition, we will determine whether heavy alcohol use is associated with abnormalities in normally observed age-related differences in these neural circuits. Our central hypothesis is that immaturity in frontal circuits and relatively increased signaling in striato-limbic structures promote impulsive decision making in late adolescents. We will test our central hypothesis via the following Specific Aims: 1) Indentify functional differences in frontostriatal circuits associated with Now/Later decision making in late adolescents versus adults. 2) Determine whether the impulsive decision-making observed in heavy drinking adults is associated with signs of functional immaturity in frontal circuits. To achieve the goals of this research plan, a previously validated delay-discounting task (Mitchell et al., 2005; Boettiger et al., 2007) will be used to measure Now/Later decision making behavior in late adolescents and adults in the context of functional MRI. We will seek to determine whether age-dependent differences in the function of brain structures of interest correlate with age-dependent differences in decision-making. These studies will be conducted in both moderate and heavy drinking populations to determine how alcohol use impacts age- related changes in brain structures engaged in Now/Later decision making. This research stands to significantly improve our understanding of the neural underpinnings of changes in decision-making from late adolescence to adulthood, which may bear on why late adolescents are at an increased risk for developing alcohol use disorders. Greater knowledge of the mechanisms underlying risk for developing alcohol use disorders may allow for the development of better treatments or interventions.